No DOW but could the Treaty ports be used?
If Dev didn't accept Churchill's offer of the north then I don't think anything would get Ireland to participate
I have to agree with that, though it's doubtful Dev believed Churchill could or would deliver on ending Partition. Is the Offer confirmed by any documents or is it simply post-war recollections of certain officials?
I also agree that initially Ireland would add little of value from its own military resources. However, the Treaty Ports would be very useful in the Battle of the Atlantic, as would the use of Foynes for seaplanes and an earlier Shannon airport.
Once the threat of German invasion has passed, perhaps US forces (rather than British?) could be allowed use of these facilities, in the same way Portugal allowed the Azores to be used? I think that's the most that could be expected and I'm not sure how a POD could be devised that could arrange it. Maybe a U.S. Guarantee for Ireland pre-PH, that could develop into a Bases Arrangement once the U.S. enters the War? Though I'm sure that would require some internal US political changes from OTL, with perhaps the Irish-American caucus demanding Roosevelt protect Ireland as the price for key bills.
Is that remotely plausible?
As I read it, Irish neutrality was to some extent biased towards the Allies, despite the rumours (unfounded I think) of U-boats refuelling in Irish bays and retaining diplomatic formalities with Nazi Germany. AFAIK Allied airmen or sailors that landed in Eire were discreetly passed over the border rather than being interned. Perhaps there's scope for other co-operation, again more with the US than directly with Britain.
To get Eire to join the War would be very difficult and IMHO need a POD much earlier than 1938. Doing away with the Civil War could be needed and also having the North less anti-Catholic. I've read a Conspiracy Theory that Churchill pressed Collins to bombard the Four Courts in order to provoke the Civil War thus reducing the IRA's ability to protect Catholics in the North and dispute the Border being drawn up. According to this, the deaths of Collins and some other leading Republicans (both sides) were also arranged by the British in order to bring Dev to power, as his autarchic Catholic Eire would not be a Revolutionary example to other Colonies.
Now I don't believe this but it would be sort of fitting if such a Churchillian Plot also resulted in Eire refusing to help Britain in a War!